The present invention generally relates to a vertical take-off and landing (VTOL) aircraft having a wing free for rotation about a spanwise axis to maintain a constant angle of attack with the relative wind. More particularly, the present invention relates to a VTOL aircraft having a free wing comprised of left and right wings projecting on opposite sides of the fuselage and mounted to the fuselage for free joint pivotal movement about a spanwise axis forward of the aerodynamic centers and controlled pivotal movement relative to one another for roll control during vertical and horizontal flight, as well as transitions therebetween.
As discussed in my prior application, a free wing or "freewing" is a wing attached to an aircraft fuselage in a manner such that the wing is freely pivotable about its spanwise axis forward of its aerodynamic center. This arrangement enables the wing to have an angle of attack which is determined solely by aerodynamic forces acting on the wing. Rotation of the wing, without pilot intervention, induced by changes in the direction of wind over the wing surfaces causes the angle of incidence between the wing and the aircraft fuselage to vary so that the wing presents a constant angle of attack to the relative wind which, in horizontal flight, enables the aircraft to be essentially stall-free.
Among other advantages realized when employing a free wing in horizontal flight are increased resistance to stalls, increased C.G. (center of gravity) range, alleviation of gust loads, e.g., on the order of a 4:1 reduction, extension of the payload capability due to the ability to reduce the structural weight of the aircraft and the ability to utilize a smaller engine with a lower fuel requirement, thus increasing flight range of the aircraft.
One the major advantages of a free wing aircraft is that the aircraft is intrinsically stable, i.e., much more stable than conventional fixed wing aircraft in response to atmospheric turbulence. In accordance with the present invention, a free wing aircraft is provided, preferably for use as an unmanned aerial vehicle where a highly stable platform is necessary and desirable, although the vehicle could be manned. For example, unmanned aerial vehicles (UAV) are often used by the military as platforms for maintaining sensors trained on a target. The sensor, for example, may comprise a video camera or a laser spot designator. UAV aircraft are highly desirable for their target standoff ability and, importantly, are usable without risk to pilots of fixed wing aircraft performing the same mission. Where generally low wing loaded (i.e., light) fixed wing UAV aircraft have been used for this purpose, their high sensitivity to turbulence, particularly at low altitudes, requires a stabilization system for onboard sensors to counter turbulence-induced platform motion. Such stabilization systems typically use gimballing, which cause the system to be heavy, complex and expensive. The high stability of a free wing aircraft eliminates or minimizes the stabilization problem in a UAV aircraft because the platform itself, i.e., the fuselage, is much more stable, even in low-altitude, highly turbulent conditions.
Vertical take-off and landing capability of UAV aircraft is also desirable. Take-off and landing sites for conventional fixed wing UAV aircraft, particularly in a military scenario, involve substantial costs which practically eliminate use of UAV aircraft from mobile launch sites, for example, small naval ships or tracked vehicles, or else require such complex launch schemes as RATO (rocket-assisted take-off). Currently, VTOL aircraft are generally in three principal categories: helicopters, tilt rotor aircraft and tail-sitters. These are highly complex and expensive. Accordingly, there is a need for a relatively simple, inexpensive UAV aircraft affording a stable platform and which aircraft has vertical take-off and landing capability.
According to the present invention, there is provided a VTOL free wing aircraft which eliminates the complexities and expense of other types of vertical take-off and landing aircraft, such as helicopters and tilt wing rotor vehicles, and provides a novel and improved VTOL aircraft affording a highly stable platform during horizontal flight without the complexities of wing-mounted engines or rotor blades and attendant structural and other problems. Particularly, the present invention provides an aerodynamic apparatus comprised of a VTOL free wing aircraft having a fuselage with left and right free wings on opposite sides of the fuselage connected one to the other for joint free pivotal movement relative to the fuselage. In accordance with another aspect of the invention, the free wings are, however, rotatable relative to one another under pilot, computer or remote control to differentially alter the angle of attack of the left and right wings while simultaneously maintaining free wing capability relative to the fuselage. That is, the left and right wings are connected one to the other and collectively are free to rotate, depending upon the aerodynamic forces applied to the wings, while simultaneously the pitch of one wing relative to the other wing may be controlled, e.g., by an actuator or a mechanical coupling between the wings or by elevons, which determine the wing pitch of each wing independently of the other. This is significant in vertical take-off and recovery procedures, as well as in horizontal flight for roll control. The VTOL free wing aircraft hereof does not require ailerons for roll control, although they may be provided for that purpose. Rather, the two wings each serve as large aerodynamic surfaces to supply whatever roll forces may be desired. It will be noted that the elevons in those embodiments which use elevons to set the pitch of the wings will not contribute to a desired roll force in most designs. Instead, they turn the wings, which then generate the roll forces.
Other components of the VTOL free wing aircraft hereof include a fuselage having a rudder for maintaining yaw control during vertical flight and an elevator, preferably adjacent the aft end of the fuselage, to assist in transitioning between vertical and horizontal flight. Either or both the rudder and elevator may comprise all moving control surfaces or a movable portion of a fixed stabilizer surface. The elevator is also useful for pointing the fuselage during horizontal flight, for example, to aid in targeting onboard sensors. It will be appreciated that other control systems for rotating the fuselage about the pitch axis may be utilized in lieu of a elevator. For example, a small canard surface adjacent the forward end of the fuselage may be used to transition the aircraft from horizontal to vertical flight and to point the fuselage. Small jets of air may also be used, but afford additional complexities which are not desirable in a UAV aircraft.
Preferably, a tractor-type propulsion system is employed in the VTOL free wing aircraft hereof, not only to facilitate launching the aircraft, but also to afford effective roll, pitch and yaw control during launching and vertical flight at minimum or zero aircraft speed. This is accomplished by allowing the propwash to serve as a source of dynamic pressure or the control surfaces. In one embodiment, the VTOL free wing aircraft of the present invention may sit vertically with the vertical and horizontal stabilizers resting on a horizontal surface prior to launch.
Should that surface be subjected to motion, or the winds which would cause the aircraft to tip over, a launch system may be used to assist in vertical take-off, principally to prevent the UAV from being tipped over by horizontal winds while awaiting launch. For example, the aircraft may be provided with suitable guides which engage a launch rail with the aircraft and rail directed generally vertically. With a tractor-type propulsion system, e.g., one or more propellers at the forward end of the fuselage, the propulsion system is at a location which does not interfere with the launch system. Additionally, the tractor propulsion system provides air flow over at least inboard portions of the free wings, enabling the free wings to react to the aerodynamic forces acting on them and thereby enable positive roll control during initial phases of flight and hover.
The aircraft may be launched in the vertical position with the rail supporting the aircraft. It will be appreciated that, at launch, the free wings are likewise oriented generally vertically and, because of their free pivotal mount, the wings weathervane, minimizing loading on the aircraft at launch. The tractor propulsion system, with possible assist from the launching system such as a catapult assist, enables the aircraft to lift off vertically. The air flow from the propulsion system, e.g., the single propeller or counterrotating propellers, causes airflow over the wings, rudder and elevator. By setting the pitch of one wing relative to the other wing under pilot, computer or remote control, control of the aircraft about the roll axis may be affected while the rudder and elevator provide yaw and pitch control during vertical flight. To transition from vertical to horizontal flight, down elevator causes the aircraft fuselage to pitch forwardly. As the direction of thrust changes, the wings will pivot freely, depending upon the relative wind, and therefore pivot toward a horizontal orientation and commence to develop lift. As the fuselage pitches toward the horizontal and horizontal speed is increased, the effect of the relative wind acting on the wing overcomes the effect of the propeller blast and the wings gradually afford lift to the aircraft principally from the forward or horizontal speed of the aircraft. When horizontal flight is obtained, horizontal free wing flight control, similarly as set forth in my prior application, is maintained.
To transition from horizontal to vertical flight, generally the reverse of the procedure noted above may be followed. For example, up elevator is given to pivot the fuselage about the pitch axis in an upward direction thereby changing the thrust vector. Forward speed slows and the air flow over the free wings eventually is dominated by the prop wash rather than the horizontal speed of the aircraft. In that vertical orientation, the VTOL aircraft can be positioned a short distance above a recovery net and the engine turned off whereby the aircraft will drop gently into the net. This is to be contrasted to the common current practice of flying horizontally into a vertical net, causing great stress on the airframe and instruments. The net may be replaced by any soft support such as foam rubber or possibly a naturally occurring support such as tall grass.
In a preferred embodiment according to the present invention, there is provided an aerodynamic apparatus comprising a vertical take-off and landing aircraft having a fuselage, a wing on each side of the fuselage having an aerodynamic center, means for connecting the wings one to the other and to the fuselage for joint free pivotal movement relative to the fuselage about a spanwise axis forwardly of the aerodynamic centers, means for selectively controllably rotating at least one of the wings relative to the other of the wings for roll control and a common propulsion system for propelling the aircraft in a vertical flight mode including vertical take-off and landing and in horizontal flight.
In a further preferred embodiment according to the present invention, in a vertical take-off aircraft having a fuselage with pitch and yaw controls and a wing on each side of the fuselage having an aerodynamic center, the wings being connected to one another and to the fuselage for joint free pivotal movement relative to the fuselage about a spanwise axis forwardly of the aerodynamic centers and being rotatable relative to one another about the axis for roll control, there is provided a method of transitioning between generally vertical take-off flight and horizontal flight, comprising the steps of propelling the aircraft in the generally vertical direction with the fuselage initially generally vertically oriented and the wings freely rotatable relative to the fuselage in accordance with aerodynamic forces acting on the wings, and actuating the pitch control to pivot the fuselage toward a horizontal orientation and thereby provide a horizontal thrust component so that the aerodynamic forces on the wings pivot the wings relative to the fuselage and provide lift for horizontal flight.
In a further preferred embodiment according to the present invention, in an aircraft having a fuselage with pitch and yaw controls and a wing on each side of the fuselage having an aerodynamic center, the wings being connected to one another and to the fuselage for joint free pivotal movement relative to the fuselage about a spanwise axis forwardly of the aerodynamic centers and being rotatable relative to one another about the axis for roll control, there is provided a method of transitioning between generally vertical and horizontal flight modes with the fuselage oriented generally vertically and horizontally, respectively, comprising the steps of propelling the aircraft in one of the flight modes with the wings freely rotatable relative to the fuselage in accordance with aerodynamic forces acting on the wings and the fuselage in the corresponding orientation, actuating the pitch control to pivot the fuselage toward its orientation in the other of the flight modes and thereby provide a thrust component in the direction of the other flight mode so that the aerodynamic forces on the wings pivot the wings relative to the fuselage toward an orientation in the other flight mode.
Accordingly, it is a primary object of the present invention to provide a novel and improved VTOL free wing aircraft and method of operation.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.